NGC 6822 WR-12
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 19h 45m 13.5s[1]
Declination −14° 45′ 12.9″[1]
Apparent magnitude (V) 18.96[2]
Characteristics
Evolutionary stage Wolf-Rayet
Spectral type WN4[3]
U−B color index -0.54[1]
B−V color index -0.17[1]
Astrometry
Distance472,000[4] pc
Absolute magnitude (MV)−5.41[3]
Details
Mass36[3] M☉
Radius3.77[3] R☉
Luminosity1,288,000[3] L☉
Temperature100,000[3] K
Other designations
LGGS J194513.50-144512.9
Database references
SIMBADdata

NGC 6822-WR 12 is a WN-type Wolf-Rayet star located in the galaxy NGC 6822, about 1.54 million light years away[4] in the constellation of Sagittarius. NGC 6822-WR 12 was the first Wolf-Rayet star to be discovered in the galaxy,[5] and is one of only four known in the galaxy.[6]

Discovery

In 1983, a Wolf-Rayet (WR) star was identified in the barred irregular galaxy NGC 6822. The appearance of strong ionised helium emission lines in its spectrum, together with ionised nitrogen emission lines but no carbon lines, led to the assignment of the spectral class WN3. At the time, it was the only known WR star in NGC 6822.[5]

NGC 6822-WR 12 was the 12th candidate of 12 candidate WR stars found in NGC 6822 during a survey of NGC 6822 and IC 1613.[7] In a follow-up study, only 4 of the WR candidates in NGC 6822 were confirmed as WR stars,[8] and they are still the only WR stars known in NGC 6822.[6]

Properties

High-resolution spectroscopy of NGC 6822-WR 12 gives a spectral type of WN4 and CMFGEN atmosphere models give a very high temperature of 100,000 K. Combined with a radius of 3.77 R☉, this leads to a bolometric luminosity of 1.3 million L☉, which would likely make it one of the most luminous stars in its relatively small galaxy. Assuming mass-luminosity relations for Wolf-Rayet stars points to a very high mass, about 36 solar masses. NGC 6822-WR 12 has a powerful stellar wind, which ejects 10−4.6 M☉ per year from its surface at a relatively slow speed of 1,100 kilometres per second.[3]

Composition

As is typical of WN stars, NGC 6822-WR 12 has almost no hydrogen, it having been either fused to helium or lost through strong stellar winds. However, due to the very low metallicity of NGC 6822, similar to that of the Small Magellanic Cloud, it has a lower nitrogen abundance than that of galactic WN stars, containing just 0.3% nitrogen. The emission lines of ionised nitrogen in the spectrum are correspondingly weak. The rest of the star is helium and its spectrum is dominated by emission lines of ionised helium.[3]

References

  1. 1 2 3 4 Massey, Philip; McNeill, Reagin T.; Olsen, K. A. G.; Hodge, Paul W.; Blaha, Cynthia; Jacoby, George H.; Smith, R. C.; Strong, Shay B. (2007-12-01). "A Survey of Local Group Galaxies Currently Forming Stars. III. A Search for Luminous Blue Variables and Other Hα Emission-Line Stars". The Astronomical Journal. 134 (6): 2474–2503. arXiv:0709.1267. Bibcode:2007AJ....134.2474M. doi:10.1086/523658. ISSN 0004-6256. S2CID 119255465.
  2. ↑ Massey, Philip; Olsen, K. A. G.; Hodge, Paul W.; Jacoby, George H.; McNeill, Reagin T.; Smith, R. C.; Strong, Shay B. (2007). "A Survey of Local Group Galaxies Currently Forming Stars. II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample". The Astronomical Journal. 133 (5): 2393–2417. arXiv:astro-ph/0702236. Bibcode:2007AJ....133.2393M. doi:10.1086/513319. S2CID 119456878.
  3. 1 2 3 4 5 6 7 8 Abbott, Jay Brian (2004-01-01). Quantitative spectroscopic studies of Wolf-Rayet stars in local group galaxies (Thesis). Bibcode:2004PhDT.......161A.
  4. 1 2 Dimitrova, Tzvetelina A.; Neugent, Kathryn F.; Massey, Philip; Levesque, Emily M. (2022-02-01). "Locating Red Supergiants in the Galaxy NGC 6822". The Astronomical Journal. 163 (2): 70. arXiv:2112.03990. Bibcode:2022AJ....163...70D. doi:10.3847/1538-3881/ac410e. ISSN 0004-6256. S2CID 244954507.
  5. 1 2 Westerlund, B. E.; Azzopardi, M.; Breysacher, J.; Lequeux, J. (1983-07-01). "Discovery of a Wolf-Rayet star in NGC 6822". Astronomy and Astrophysics. 123: 159–161. Bibcode:1983A&A...123..159W. ISSN 0004-6361.
  6. 1 2 Neugent, Kathryn; Massey, Philip (2019-08-01). "The Wolf-Rayet Content of the Galaxies of the Local Group and Beyond". Galaxies. 7 (3): 74. arXiv:1908.06238. Bibcode:2019Galax...7...74N. doi:10.3390/galaxies7030074.
  7. ↑ Armandroff, T. E.; Massey, P. (1985-04-01). "Wolf-Rayet stars in NGC 6822 and IC 1613". The Astrophysical Journal. 291: 685–692. Bibcode:1985ApJ...291..685A. doi:10.1086/163107. ISSN 0004-637X.
  8. ↑ Massey, Philip; Conti, Peter S.; Armandroff, Taft E. (1987-12-01). "The Spectra of Extra-Galactic Wolf-Rayet Stars". The Astronomical Journal. 94: 1538. Bibcode:1987AJ.....94.1538M. doi:10.1086/114586. ISSN 0004-6256.
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